While modern compound bows are quieter, more accurate, and more vibration-free that we could have imagined just 5 years ago, it seems as if compound bow speeds may have hit a technical rut. Today’s IBO speeds aren’t so different than those of 5 years ago. A soft-cam target bow shoots about 280 or 290 fps. A basic hunting bow shoots roughly 300-310 fps. An “en vogue” high-performance compound often squeezes into the 315-325 fps range, and a few radical designs venture into the 330’s and 340’s.
So what’s the problem?
Why aren’t bows getting faster and faster as technology continues to improve? Will, we ever see a 400 FPS IBO Speed compound bow, or is that goal simply unattainable from a hand-drawn weapon?
Arrow Mass Standards
Can any bow shoot 400 fps today?
Sure! It just can’t be done safely or within accepted industry standards. If you shoot a light enough arrow, your high-performance compound bow can surely make 400 fps. It just won’t do it for very long. Shooting underweight arrows put enormous stress on a bow’s components – much like dry-firing the bow.
If you shoot dramatically underweight arrows, you may eventually warp the cam tracks, bend the cam axles, damage the string and/or buss cable, or get catastrophic limb failures (just to name a few potential hazards).
So it would be particularly unwise to “cheat” your way to faster arrow speeds by ignoring industry standards. While every expert doesn’t agree on this issue, the most commonly accepted arrow mass standard is the IBO (International Bowhunting Organization) Standard which states that minimum arrow mass must be at least 5 grains per each pound of draw weight. So to meet the safety standard, a 70# bow must shoot an arrow that weighs at least 350 grains (70×5).
The IBO also has a standard for speed-testing bows. Since it would be confusing for one manufacturer to test and rate their bows using one standard, and another using a different standard, manufacturers generally rate their bows using the same IBO method. To get an accurate IBO Speed rating, manufacturers must test their bows under the same preset conditions: setting the bow for exactly 70# Peak Draw Weight, exactly 30″ Draw Length, and they must shoot a test arrow that weighs precisely 350 grains. This way consumers have a fair apples-to-apples method of comparison.
AT The Drawing Board
OK, so let’s start at the top. First, we need to know, is it theoretically possible for a bow to shoot 400 fps at IBO test standards? Surprisingly, the answer is yes. However, a compound bow engineered for the “theoretical maximum” would have a totally flat Powerstroke which pulled at peak weight from the first inch to the last – with no let-off. And to actually reach the theoretical performance limit, the bow would need to be 100% efficient (today’s bows are typically 75-85% efficient). But let’s take a look at the on-paper possibilities.
Using the archer’s Kinetic Energy formula, KE = (mv²)/450240, we can compute that a 350-grain arrow traveling at 400 fps would carry 124.38 ft-lbs of KE. So the first step is to see if it’s mathematically possible to store that much energy during a 70#/30″ powerstroke. After all, if you can’t put that much energy into the bow (with muscle effort), you could never hope to get that much out (energy into the arrow). So let’s start by looking at energy input.
A bow with a 6″ brace height and a 30″ draw length will have a 22.25″ Powerstroke. If the draw weight remains constant at 70 lbs. for the entire Powerstroke, that bow could potentially store up 1557.5 in-lbs (22.25 x 70) or 129.79 ft-lbs (1557.5/12) of energy. So it is possible for a powerstroke to store the minimum 124.38 ft-lbs. In fact, any bow with a brace height up to 6.93″ could theoretically store the required energy to achieve 400 fps with a 350-grain arrow.
However, note again that this is assuming a totally flat power curve (pulling 70# all the way back with no ramp-up or ramp-down of draw weight during the cycle). This also assumes 100% efficiency (more on this in a moment), such that the bow’s output precisely equals its input.
So it is theoretically possible. But could it be done while still maintaining let-off and a tolerable Powerstroke? And could bow efficiency be boosted high enough to get the required output? To understand this issue, we’ll first need to understand the basics of the Force Draw Curve. The force draw curve is a simple graphic representation of how a compound bow’s draw weight changes as it is drawn back and then let back down (draw cycle). Careful examination of this graph can tell us a lot about how a compound bow will feel and perform.
Reading The Humps
Take a look at the graph on the right. This graph represents drawing-back a compound bow and then letting it back down. Length (distance) is plotted against weight. At the marker, the bow is at rest. At marker, the bow has been drawn back about 4 inches and the draw weight has increased to roughly 40 lbs. As the shooter continues to draw back, the weight gradually increases until reaching the bow’s peak weight (roughly 67# in this example) during the 10th inch of the powerstroke at marker. Then the draw weight begins to decrease until finally reaching full let-off at marker representing the end of the powerstroke (full draw). The spike in the middle of the graph represents forcibly overdrawing the bow (pulling against the wall). Some shooters tend to hold hard against the wall, others don’t. So the spike in the middle could be different (taller/shorter) depending on the shooter.
The second half of the graph represents letting the bow back down (or firing the bow). Notice that the graph is almost a reverse of the powerstroke, except all the draw weight values are slightly lower.
It’s All in The Geometry
The sample graph above is taken from a moderate bow (PSE Bruin) with a relatively smooth-drawing cam. Notice that the overall shape of the graph is a smooth bell-shaped curve with a gradual rise and gradual decline. Interestingly, the general shape of the curve is a good estimate of how aggressive the draw cycle will feel to the shooter. And as you might expect, all cams are NOT created equal. Some cams are specifically engineered to produce a smooth feel. Others are made for the best possible performance. The actual geometry of the cam system determines how soft or aggressive the powerstroke will be. Take a look at the additional sample graphs below, taken from bows with different types of cam systems.
ROUND WHEEL: As you can see, a Round Wheel style bow has a very smooth bell-shaped curve which rises to peak weight for only a moment then gradually descends to full let-off. This cam style will feel very smooth and easy to draw, but will store the least amount of energy and shoot the slowest. Although this type of cam has been around for decades, some shooters still prefer the soft feel of this style cam – particularly instinctive-shooters and finger-shooters. So a number of manufacturers still offer bows with traditional round wheels or cam geometry ground to replicate the round wheel powercurve.
MEDIUM CAM: The Medium Cam graph is typical of today’s basic single and hybrid cams. These cams are more aggressive, ramping to peak weight more quickly and then coming to full let-off more abruptly. So they tend to store up more energy than a Round Wheel bow, and shoot notably faster. However, a Medium Cam is sure to “feel” a little heavier than a Round Wheel bow of equal peak weight. This type of cam geometry suits most shooters well, offering a reasonable blend of feel and performance.
HARD CAM: The last example is a Hard Cam system, optimized for maximum energy storage and speed. Notice how quickly the bow ramps up to peak weight and how quickly it transitions to let-off. Also notice the distinct high-plateau on the graph where the shooter must draw the bow over several inches at peak weight. This type of cam geometry will store dramatically more energy, and will usually have an IBO Speed of 320 fps or more. The downside is that Hard Cams feel harsh and heavy compared to other bows of equal peak weight. So they certainly aren’t for everyone. But for shooters who want the hottest possible arrow speeds, the Hard Cam is the way to go.
NOTE: Since many shooters associate a “round” cam with a “smooth” cam, a few bow manufacturers deliberately machine their cams to be round in shape, although the cam generates a very aggressive hard-cam cycle. Don’t be fooled by this little mind-trick. The “power side” of a single cam is the center groove – NOT the outside track. So don’t automatically assume a round cam is a smooth cam with the same draw characteristics as a round wheel. They’re not the same thing.
This is a comparison graph, taken from a popular manufacturer’s catalog. Notice how this particular manufacturer offers 4 distinctly different cam grinds. The pink line represents the least aggressive grind (smoothest) and the blue line represents the most aggressive grind (fastest). The white line indicates the standard (most popular) grind which offers a moderation of the two extremes. Of course, there is no right or wrong. But manufacturers are smart to offer buyers a choice, as each shooter has different expectations. So most bow companies offer both soft and aggressive cam grinds to try and suit everyone.
NOTE: This graph looks a little different because it only shows the front half of the draw cycle (which is typical). Our in-house graphs are generated with Easton’s Bow-Force Mapping System which can show both halves of the draw cycle.
Area Under The Curve
So what do all these lines and curves really mean when it comes to getting to 400 fps? Everything! As cam aggression increases, so does stored energy. On the Force Draw Curves, the input energy is represented by the areas under the curve. The more area, the more energy stored, and the more speed the bow can yield when fired. As you can see on the graphs below, the Hard Cam Bow has the largest area under the curve (represented in blue). So any design that’s going to make 400 fps will have to take the Hard Cam concept to the extreme.
As the Hard Cam bow becomes more and more aggressive, the graph begins to look less like a curve, and more like a rectangle. In fact, at the theoretical maximum, the draw force curve wouldn’t be a curve at all. If a bow maintained a constant 70# peak weight throughout the entire powerstroke, the resulting area under the “curve” would be a 22.25″ x 70# block (assuming a 6″ brace height and a 30″ draw length). But of course, the bow would have no let-off, rendering it virtually unshootable.
Without at least some let-off, developing the 400 fps super-bow wouldn’t be very realistic. So we’ll need to incorporate some minimal let-off into the super-bow draw cycle to make the bow usable. In our optimized example, we’ve adjusted the curve to incorporate 65% let-off during the last 2″ of the powerstroke, reaching 65% relief at exactly 30″ draw. This reduces our energy input by 45.5 in-lbs or 3.79 ft-lbs.
Ramp Up To Peak Weight
The other obstacle would be how quickly the bow could ramp-up to peak weight at the start of the draw cycle. During the initial pull, you effectively change a straight string into a bent string (at the nocking point roughly), but the angle begins at 0º. So until the angle increases enough to rotate the cams, the weight cannot ramp-up. Even today’s most aggressive cam designs require about 4″ of drawstroke before coming up to peak weight. But on the 400 fps super-bow, we’re going to assume engineers have managed to get that down to an insanely abrupt 1″. So we’ve adjusted the optimized curve to allow for some ramp-up time. Of course, this reduces our energy input by another 35 in-lbs or 2.92 ft-lbs.
Best Case Energy Storage
The powerstroke on the 400 fps super bow would represent the most aggressive possible curve, while still maintaining at least some field-utility and shootability. So before we discuss bow efficiency, let’s take a look at the numbers and see if we theoretically still have enough energy to get the job done.
Maximum Energy Storage
Theoretical Max Storage
Super-Bow Max Storage
If you remember, we’re going to need 124.38 ft-lbs. to push a 350 grain arrow at 400 fps. So even after making changes to incorporate let-off and limited ramp-up length to the super-bow powerstroke, it is still possible to hit the 400 fps mark.
Unfortunately, the 400 fps super bow will now require a wrist-blistering sub-6″ brace height to hit the energy minimum. However, it is to be expected that the ultimate speed-bow would have a short brace height. So the 400 fps bow is still a possibility at 100% efficiency.
Energy In And Energy Out
You may remember noting that the second half of the Force Draw graph is almost a reverse of the first half (powerstroke), except all the draw weight values are slightly lower on the way back down. This is one of the unfortunate realities of any machine. No matter how good a machine is, you can never get more energy out than you put in. In fact, you always get less.
Of course, the energy isn’t lost, it just gets converted into other things we don’t necessarily want (heat, vibration, noise, etc.). For a compound bow, the goal is to essentially convert muscle energy into propelling an arrow forward. And if the system were 100% efficient, then the arrow would leave the bow with the same amount of energy used to draw the bow back. Unfortunately, the reality is somewhat different. Even the best bows on the market are well shy of being 100% efficient.
The Energy That Got Away
As you draw a bow, some of your muscle energy is used to overcome friction in the system rather than just to compress the bow’s limbs. Friction in the cam axles, string surfaces, cable slide, etc. all add a little draw weight to the cycle. So when you draw your 70# bow, you’re expending a few pounds of effort just to make things “turn”. Sadly, you don’t get this energy back when you fire the bow. This is why the output side of the Force Draw Curve is always a little lower than the input side. This degradation or loss of effective draw weight due to friction forces is called hysteresis, and it’s something that even the 400 fps super bow will need to contend with.
So even if we manage to get the 124.38 ft-lbs of input energy into the bow, we’re going to get some lesser amount out. The question is, how much will we lose? To help visualize the concept, the graph at right is the same as the system efficiency graph above, but it has been folded in-half to allow you to compare the two curves. The area between the two curves (in dark blue) represents the energy that is lost to hysteresis.
While a good high-performance bow is around 80% efficient, there are a few bows that really lead the field. The new Bowtech Guardian, for example, boasts efficiencies in the 86-88% range. So this is clearly an area where all manufacturers can improve. We submit that a 95% efficiency goal is attainable before the end of the decade.
Super Bows Of Today
So how close are we now? Over the last few years, several bow manufacturers have laid claim to having the “World’s Fastest Bow” with 340+ IBO Speeds. So we’re realistically within 50-60 fps of the theoretical Super-Bow. And it does seem that average IBO Speeds are slowly creeping up each year. So bow manufacturers are making some progress. However, 50-60 fps would represent quite a quantum leap.
Industry Average IBO Speeds
Mathews Black Max II Turbomax
Bowtech Black Knight II
APA Black Mamba Extreme
APA Black Mamba X1
APA Black Mamba X2
The 400 FPS MARKS
So, can we get realistically there? Can there be a 400 fps Super-Bow in our near future? Take a look at the chart below. If we use the Archer’s KE formula, with known values for KE and arrow mass, we can solve for arrow velocity and estimate the IBO Speeds of our theoretical Super-Bow, given a particular mathematical efficiency. As you can see, the answer is yes – but just by a fine serving thread. With an ultra-aggressive cam system, an anorexic brace height, and a wildly improved total system efficiency, a 400 fps Super-Bow could someday be built.
Theoretical Max Input
IBO Speed @ 85% Effic.
IBO Speed @ 90% Effic.
IBO Speed @ 95% Effic.
IBO Speed @ 100% Effic.
The Super Bow Market
But who would shoot the Super Bow? No matter how it was marketed, the Super Bow would ultimately be an uncomfortable ultra-aggressive bow with a wrist-popping short brace height. So even if it was achieved, it’s unlikely the Super Bow would be more than a novelty in the archery market? In fact, the 340+ fps super bows of today tend to be slow sellers that don’t attract much attention after the chronograph is switched-off. So while everyone appreciates good performance, it’s clear that shooters generally prefer more moderate designs with 7″+ brace heights and comfortable power strokes. So the Super Bow’s future may be more about the possibilities of research and development than actually trying to market and sell the first 400 fps bow.
Since shooters tend to turn their backs on ultra-aggressive drawstrokes, developing hotter powercurves may not be the answer. We submit that the real future of high-performance bow technology will be about tweaking efficiencies. Just a 5% increase in efficiency would add over 15 fps to the average IBO Speed – without making drawstrokes unnecessarily uncomfortable. This 5% increase would make today’s popular bows all seem like super bows. And should efficiencies ever squeak into the 95% range, even a moderate bow may someday shoot well into the 340’s or 350’s. But for now, the 400 fps Super Bow will just have to remain a speed-junky’s fantasy.